1. Field of the Invention
This invention relates to improvements of coating type magnetic recording mediums having magnetic layers which have excellent corrosion resistance, and to binders for use with the magnetic recording mediums. This invention, particularly, relates to the magnetic recording mediums capable of high density recording without degradation of mechanical strength and durability, and the binders having excellent dispersibility of the magnetic particles.
2. Description of the Related Arts
In the prior art, as a binder for binding magnetic particles on a supporting base of the magnetic recording medium, there have been used binders combined with various kinds of resin. However, these binders have less corrosion resistance for protecting the magnetic particles such as metal magnetic particles or a metal magnetic powder from oxidation, which is used for high density recording. Thus, an attempt has been done to enhance the corrosion resistance of the magnetic medium by employing surface treatment of the metal magnetic particles. But this has not brought an adequate result.
Recently, for eliminating the above problem, there are being developed binders capable of improving corrosion resistance of the metal magnetic powder by introducing aminoquinone compounds to a resin component of the binder.
The binders introduced with the aminoquinone compounds (referred to as aminoquinone binder) have excellent corrosion resistance, however, they can not produce its inherent advantage adequately because the dispersibility of the magnetic powder is degraded in the binders, in particular, when the magnetic powder is made of small sized particles.
Further, the study of improvement of the magnetic recording characteristic is being made in the recent magnetic recording mediums so as to realize the high recording density. Specifically, the study of surface smoothness of the magnetic recording medium is forwarded to prevent the degradation of reproducing output level developed due to a spacing loss between a magnetic gap of a magnetic head and the magnetic recording medium as the study of providing a saturation magnetic flux density Bm and the coercive force Hc of the magnetic recording medium are forwarded. However, the present situation is that it is very difficult to obtain an adequate surface smoothness from the magnetic recording medium employing the aminoquinone binder.
Furthermore, when such aminoquinone binders are used, the mechanical strength of the magnetic recording layer is largely degraded, so that the mechanical strength of the magnetic recording medium itself is also degraded largely.
As mentioned above, though the aminoquinone binders in the prior art are excellent to prevent metals from being corroded, but they does not satisfy the dispersibility of the magnetic powder, the surface smoothness and mechanical strength of the magnetic layer.
In the coating type magnetic recording medium, the magnetic layer is formed by coating a magnetic paint containing a mixture of the binder and the magnetic powder on a supporting base.
In the prior art, in order to improve the electro-magnetic conversion characteristic, an attempt was made to improve the S/N (a ratio of a signal output level to a noise level mainly caused by a particle noise) by employing the magnetic powder having a small particle size. Actually, good results have been obtained from that because the particle noise depends on the diameters of the magnetic particles. However, as mentioned in the foregoing, the smaller the diameters of the magnetic particles become, the more difficult the dispersion of the magnetic particles into the binder (binding resin) becomes. Thus, it was impossible to obtain a desired electromagnetic conversion characteristic as long as employing the conventional binders which have been widely used.
An attempt has been made to resolve the drawback, i.e., the difficulty of the dispersion of the magnetic particles, by introducing a functional group having a high polarity into a resin component of the binder.
Upon the industrial production of the magnetic recording mediums, it is a very important subject to coat the magnetic paint on the supporting base maintaining a stable condition of the magnetic powder well dispersed in the binder. But, the smaller the diameters of the magnetic particles, the more the dispersibility of the magnetic particles is degraded, thus in the prior art, it was unable to obtain the magnetic paint capable of maintaining the stable condition of the magnetic powder well dispersed for a long time. In particular, when a crosslinking agent was added to the magnetic paint to secure durability of the magnetic layer, the cohesion of the magnetic particles was accelerated, resulting in increasing a paint viscosity, thus, degrading the dispersibility thereof. Particularly, the sizes of the magnetic particles of less than 0.1 .mu.m caused serious problems, which posed obstacles to the industrial production of the magnetic recording mediums. Further, when the particle sizes of the magnetic powder are minimized, the dispersion stability of the magnetic powder tends to be deteriorated corresponding to a specific surface area of the magnetic powder. Particularly, in the case of adding the crosslinking agent into the magnetic paint to secure the durability of the magnetic layer, the cohesion of the magnetic particles is enhanced, resulting in increasing the paint viscosity, as mentioned in the foregoing. The specific surface area of not less than 45 m.sup.2 /g in the magnetic powder causes serious problems, though the larger specific surface area of the magnetic powder is advantageous to the electro-magnetic conversion characteristic of the magnetic recording medium.
Recently, according to the demand of a high density recording, there has been used the metal magnetic powder (metal ferromagnetic powder). Further, for a practical use of a short wavelength recording of not more than 0.5 .mu.m, the metal magnetic powder, each of magnetic particles having a long axis of not more than 0.1 .mu.m and a saturation magnetization (.sigma..sub.s) of not less than 130 emu/g requires to be utilized for recording.
Further, instead of the conventional analogue magnetic recording, the digital magnetic recording comes to the main current of the magnetic recording. Thus, the thickness of the magnetic layer needs to be about not more than 0.2 .mu.m because excellent over-write characteristic is required.
Further, there is a problem of durability of the magnetic recording medium, in particular, of a magnetic tape when the magnetic layer is required to be made much thinner so as to increase the volume recording density and the playing time of the magnetic tape in the digital recording.
As well known, the stiffness of the magnetic tape is proportional to the third power of thickness of the magnetic tape. Thus, thinning the magnetic tape signifies the substantial degradation of the tape stiffness. In order to increase or maintain the stiffness of the magnetic tape, it is necessary to enhance the mechanical strengths (Young's modulusli) of the constitutional components such as a magnetic layer, a base film and a non-magnetic layer (for instance, a back coat layer). However, it is difficult to enhance the strength of the base film because the mechanical strength thereof is limited by the material and the production method of the base film. Thus, it is advantageous to enhance the strengths of the magnetic layer and the non-magnetic layer.
The degradation of the mechanical strength of the magnetic layer decreases running durability of the magnetic tape, particularly in a still motion picture mode on a video tape recorder where the magnetic heads repeatedly run on the same tracks on the magnetic tape.
Further, the degradation of the mechanical strength of the magnetic layer invites the degradation of conservation durability such as heatproof durability, resulting in decreasing the reproducing output level of the magnetic tape and increasing dropout thereof resulting from surface defects such as tape cramp marks and print-through of the back surface of the magnetic tape when they are stored under a condition of high temperature and high humidity.